"A history of western philosophy" by Bertrand Russell. The world's most prominent philosopher at the time, took a few years out of his working at the front line to write a summary on 2000 years of thought for layman's and won a nobel prize. It's not the easiest read, you can take issue with some interpretations but theres nothing like it. I think it required reading for being a thoughtful human.

“A precious book … a work that is in the highest degree pedagogical which stands above the conflicts of parties and opinions.” – Albert Einstein